Since the discovery of carbon nanotubes (“CNTs”) by Sumio Iijima in 1991, CNTs have gained immense popularity in the field of nanocomposites by exhibiting an unprecedented combination of beneficial mechanical, thermal and electrical properties. The use of CNTs as nano-fillers in polymer composites has demonstrated the potential for improvements in mechanical properties such as tensile and compressive strength, elastic modulus and fatigue resistance, in addition to enhanced thermal and electrical properties. More recently, polymer/CNT nanocomposites have received extensive recognition for the versatility they offer in a variety of applications such as water purification, gas sensing, strain sensing, super capacitance, fuel cell electrodes, fire retardant coatings, artificial muscles, EMI shielding and self-heating hybrid composites for de-icing. These applications employ CNTs in the form of a thin porous membrane of highly entangled CNTs held together by van der Waals forces, sometimes referred to as “buckypaper”.
The current fabrication methods for CNT membranes are often complex, require long processing times, and impose significant size limitations on the membrane. As a result, the applications of carbon nanotube (“CNT”) membranes are still in their infancy, and prototype feasibility is often limited to laboratory scale demonstrations. A more rapid, large-scale manufacturing technique is critical to extend the application of these multifunctional capabilities to the industrial scale.
Vacuum filtration is currently used used by many researchers for manufacturing CNT membranes. This method involves vacuum assisted filtration of a homogeneously dispersed CNT solution using a Polytetrafluoroethylene (PTFE) or nylon filter with sub-micron sized pores. CNTs are deposited on the filter surface and form a thin CNT membrane (hereinafter referred to as “CNT membrane”). The CNT membrane can be lifted from the filter surface after drying and used for applications such as water purification (see for example Dumée L. F, Sears K., Schütz J., Finn N., Huynh C., Hawkins S., Duke M., Gray S 2010, “Characterization and evaluation of carbon nanotube bucky-Paper membranes for direct contact membrane distillation”, Journal of Membrane Science 351, 36-43.) This technique has been used for manufacturing CNT membranes as an embedded strain sensor in epoxy dog-bone specimens (see for example Rein M D, Breuer O., and Wagner H D 2011, “Sensors and sensitivity: carbon nanotube buckypaper films as strain sensing devices”, Composites Science and Technology, 71, 373-381), and as a smart skin for strain sensing in aircraft wings (see for example Dharap P., Li Z., Nagarajaiah S., and Barrera E V 2004, “Nanotube film based on single-wall carbon nanotubes for strain sensing”, Nanotechnology, 15, 379-382), and as a de-icing glass fiber reinforced polymer (“GFRP”) nanocomposites (see for example Chu H, Zhang Z., Liu Y., and Leng J 2014, “Self-heating fiber reinforced polymer composite using meso/macropore carbon nanotube paper and its application in deicing”, Carbon 66, 154-163.)
The use of vacuum filtration methods tends to limit the size of the manufactured CNT membranes to the diameter of the filter being used, and can also lead to heterogeneous distribution of CNT bundles in the finished product (see for example Cherng-Shii Y., “Characterization of nanotube buckypaper manufacturing process”, 2004. Electronic Theses, Treatises and Dissertations. Paper 420.) Further, the use of filters with larger diameters may not be practically feasible because of the large volume of CNT solution that would need to be filtered. Moreover, maintaining homogeneous dispersion in a large volume of CNT solution is a challenging task due to the tendency of CNTs to form agglomerates/clusters. Filtering larger quantities of solution through sub-micron filters can take several hours depending on the diameter and pore size of the filter, the volume of solution being filtered, and the pressure difference applied by a vacuum pump.
Current CNT membrane fabrication methods typically require the use of surfactants, organic binders, and/or chemical functionalization of the CNTs to assist in obtaining a stable uniform dispersion during filtration, any one of which can affect the properties of the CNTs in the final product. For example, following filtration, the surfactants can be difficult to remove from the CNT membrane, and can hamper the efficiency of the membrane for applications such as water purification, gas separation and bio fuel cell electrodes. Chemical modification of CNTs prior to the filtration can also degrade the functionality of CNT membranes for the aforementioned applications. Other less commonly used fabrication methods for CNT membranes include hydro-entanglement, which involves impregnation of high speed water jets onto CNTs present on a porous substrate, where the high pressure of water jets induces entanglement of the CNTs in the membrane (see for example Zhang X 2008, “Hydroentangling: A novel approach to high-speed fabrication of carbon nanotube membranes”, Adv. Mater. 20, 4140-4144). Not only is this method complex to set up, the use of high speed water jets demands a high power consumption, and thus is limited to the production of smaller size membranes.
Accordingly, there exists a need for fabrication methods for CNTs membranes that enable the formation of larger CNT membranes with production scale efficiency and speed, while reducing or substantially eliminating chemical and physical degradation of the CNTs.